A photoreactive catalyst, commonly referred to as a "photocatalyst," is a collection of photocatalytic particles. Slurries result from the mixture of a contaminated fluid with a photocatalyst. Exciting the photocatalyst (for example, with light of sufficient energy) creates the formation of electrons and holes on the surface of the photocatalyst. Electrochemical modifications to the contaminated fluid result from such formation. Such electrochemical modifications are generally referred to as a "photocatalytic reaction." Photocatalytic reactions are employed for numerous purposes, such as decomposition, photosynthesis, the oxidation of contaminants, the reduction of contaminants, the sterilization of bacteria, deposition of metals, and the like. For example, a photocatalytic reaction can serve to oxidize toxic organics into carbon dioxide and water.
A catalytic action results when a catalyst lowers the "activation energy" that is required to complete a chemical reaction. In photocatalytic reactions, activation energy is provided by the photon energy of incident band-gap light. Incident band-gap light is absorbed by a photocatalyst, electron and hole charge carrier pairs are produced within the photocatalytic particles. These charge carriers then induce reduction/oxidation ("redox") reactions. These reactions have the effect of destroying contaminants found in the contaminated fluid so as to render a decontaminated effluent.
Once a photocatalytic reaction has taken place, and contaminants are destroyed from the contaminated fluid, it is necessary to separate the decontaminated effluent from the photocatalytic particles. Two conventional filtration techniques are frequently used in this regard, namely, dead-end filtration and cross-flow filtration.
Both dead-end and cross-flow filtration techniques each utilize filter units that include one or more "filters" in order to separate the particles of a fluid by size differential. Such filters have pores that extend through the walls of the filter. Once a slurry is subjected to photocatalytic treatment, the resulting mixture (referred to as a "treated slurry") includes relatively large sized photocatalytic particles and smaller sized decontaminated effluent molecules. Dead-end and cross-flow filters only allow for the decontaminated effluent molecules to pass through the filter walls while precluding larger sized photocatalytic particles from passing through the filter walls. This is accomplished through the selection of the size of the pores that are disposed through the filter walls--pore sizes are selected to be smaller than the photocatalytic particles but larger than the decontaminated effluent molecules.
In dead-end filtration, a mixture of decontaminated effluent and photocatalytic particles is directed towards a filter wall with pores formed therethrough. Decontaminated effluent molecules pass through the pores disposed in the filter wall while photocatalytic particles remain and collect on the filter wall. It is both time consuming and inefficient to continuously remove the photocatalytic particles from the dead-end filter wall.
In cross-flow filtration, two directional components are employed. First, a substantial linear velocity is maintained through the filter, i.e., parallel to the filter wall. Second, a substantial pressure differential, also referred to as a transmembrane pressure, is applied across the filter. Such transmembrane pressures can exceed 100 psi. Accordingly, the high transmembrane pressure drives some of the smaller decontaminated effluent molecules through the pores disposed in the filter wall while the high linear velocity continually removes most photocatalytic particles or contaminants away from the filter wall. This inhibits the build-up of materials on the filter wall. When compared to a dead-end filter, a cross-flow filter provides for improved separation technique since photocatalytic particles do not collect on the filter wall.
Selection of pore sizes are critical to ensure the recovery of decontaminated effluents in both dead-end and cross-flow filters. For example, when employing TiO.sub.2 photocatalytic particles in the treatment of contaminated water, the primary particle size of a TiO.sub.2 particle is approximately 21 nm. (primary particle size is a measure of the average diameter of a single particle). Accordingly, the pore size necessary to separate the TiO.sub.2 particles must be less than 21 nm. However, ultrafiltration occurs when pore sizes less than 21 nm. are used to separate decontaminated water molecules from TiO.sub.2 particles having an average size of 21 nm. Ultrafiltration, in combination with a high transmembrane pressure, makes it very difficult to efficiently separate water molecules from the TiO.sub.2 particles. For example, when 100 liters of a decontaminated effluent are passed through a conventional cross-flow filter (as part of a treated slurry), 90 liters of the decontaminated effluent slurry typically exits the cross-flow filter while only 10 liters of decontaminated effluent permeate through the filter walls for recovery. Thus, only 10% of the decontaminated fluid is recovered. Therefore, the rate of recovery of decontaminated effluent through the walls of conventional cross-flow filters, referred to as "flux," is minimal.
"Foulants", such as sand, clay, oil, silt and cellular fibers, are often found in contaminated fluids. However, most foulants are not destroyed by photocatalytic treatment. Accordingly, they exist in the treated slurry and are passed into conventional filters. Upon entering a conventional filter, the foulants may adversely affect the operation of the filter and potentially degrade the filter over time. For example, a foulant may lodge in the pores of a cross-flow filter due to the high transmembrane pressure. Chemical cleansers are often required to remove foulants from a cross-flow filter. Yet, cleansing of conventional the filters is not always successful.
In summary, several disadvantages are encountered when a treated slurry is passed through either a dead-end or cross-flow filter. First, the flux through the pores of conventional filters is minimal. This translates into the minimal recovery of decontaminated fluid from a treated slurry. Second, foulants typically adversely affect and degrade conventional filters.